The present invention relates to a valve mechanism for an automotive engine and more particularly to a valve mechanism where the valve lift and valve timing of the valves are varied in accordance with engine speed.
In an automotive engine, the valve timings of the intake and exhaust valves have a large influence on volumetric efficiency of the engine, stability of low engine speed and high engine speed performance. Namely, if the valve overlap period wherein the intake valves and the exhaust valves are both opened before and after the top dead centers becomes long, the quantity of induced air increases in a high engine speed range because of the inertia of the intake and exhaust air. Thus, the volumetric efficiency is improved, and hence the power of the engine is increased. However, in a low engine speed range, such a long valve overlap period may cause the intake air to flow back or to escape to the exhaust port so that the engine speed becomes unstable. Thus, it is preferable to decrease the overlap period in the low engine speed range. Accordingly, it is required to properly control the valve timing in the entire engine speed range to ensure the stability of engine speed and high engine performance under any engine operating conditions.
In order to change the valve timing, there has been proposed a valve mechanism where a low-speed cam for a low engine speed range, a high-speed cam for a high engine speed range and rocker arms corresponding to the respective cams are provided. One of the rocker arms is operatively connected to the valve and both rocker arms are connected or disconnected with each other in accordance with the engine speed so as to select a necessary cam. Hence the valves are operated to open at a small lift and a small opening degree at a low engine speed and to open at a large lift and a large opening degree at a high engine speed.
Japanese Patent Application Laid-Open 62-32206 discloses such a valve mechanism where the rocker arms are mechanically connected. One of the rocker arms, which are mounted on a rocker arm shaft, is operatively connected to the valve. A slipper is provided between the rocker arms and engages with the low-speed cam and the high-speed cam. A changeover device having a piston and urged by a spring is provided so that the rocker arms are connected or disconnected with each other by hydraulically operating the piston.
In the system, both of the high-speed cam and the low-speed cam are engaged with the corresponding rocker arms through the slipper. Since a film of oil is not sufficiently formed between the slipper and the cams in the low engine speed range, torque for driving the rocker arm becomes extremely large as shown in FIG. 9. As a result, the fuel consumption increases. In addition, the construction of the connecting device becomes complicated because the connecting device is provided in the rocker arms. Furthermore, a shearing force is exerted on the piston when the connected rocker arms swing, which will decrease the durability of the piston.
Thus, it is preferable to employ a roller follower instead of the slipper to decrease the friction between the low-speed cam and the rocker arm, and hence to decrease the driving torque. However, since the periphery of the roller follower is located higher than the slipper, the cam shaft must be located at a higher position, which results to increase the space for the valve mechanism. Alternatively, the roller follower may be disposed in a groove formed in the rocker arm so as to reduce the hight of the roller from the rocker arm. In such a case, design of the construction and the arrangement of the changeover device must be difficult. Thus, if the roller follower is used, it is necessary to improve the changeover device.